The gas exchange responses of big bluestem (Anropogon gerrardi) were followed after defoliation by either cattle grazing or clipping and compared with the response of nondefoliated (control) plants. Grazed plants had significantly higher rates of photosynthesis than either clipped or control plants. The photosynthesis/transpiration ratio as well as stomatal sensitivity to humidity indicate that leaves of grazed plants may have developed in a higher light and lower moisture environment than that of their clipped counterparts. Although the experimental design could not preclude any indirect effects of animal activity (saliva, waste products, or trampling) on the grazed plants, the microenvironmental differences caused by grazing may be crucial in determining the responses of grazing to clipping versus grazing.